The present invention relates to an image-forming lens formed from lens elements having spherical or planar surfaces and, in particular, relates to an image-forming lens to be used as a photographic lens with TV telephones, door phones, video cameras or digital cameras.
In recent years great use has been made of solid state photographic image elements (e.g., CCD arrays) with video cameras or digital cameras. In the course of technological progress, solid state photographic image elements have come to be miniaturized over the years, and at the same time, through miniaturization in photographic lenses, greater use has been made of wide angle lenses.
On the other hand, with both video cameras and digital cameras, there has been a trend of increasing use of low pass filters to prevent moire effects, infra-red light shielding filters to correct spectral sensitivity of the solid state photographic image elements, and glass covers to protect the surface of the solid state photographic image elements. In such instances, it has been necessary to increase the back focal length Bf (hereinafter referred to as simply the back focus) of the photographic lens to some extent in order to provide adequate space between the photographic lens and the solid state photographic image elements.
Japanese Patent Publication No. 5-264895 discloses an image-forming lens which employs four lens elements having spherical or planar surfaces. And, U.S. Pat. No. 4,525,038 discloses an image-forming lens which employs five lens elements having spherical or planar surfaces. However, in these instances the picture angle is a relatively small 90 degrees. And, in the case of the latter patent, the F# is greater than 4.0, resulting in an extremely dark image.